Wadsworth.] 132 [November 1, 



bly belongs ; and to the latter of which the Somerville argillites 

 (" Cambridge slates ") are assigned, at least until further evidence 

 can be obtained. 



The proof of the unconformable superposition of the conglom- 

 erate upon the argillite in Newton, and the greater age of the 

 latter has been overlooked by Mr. Crosby, who, in conformity 

 with his theoretical views that the argillite was the younger rock, 

 explained the structure by the supposition of a closely folded 

 synclinal ; 1 — a supposition which the evidence above given pre- 

 cludes. 



Whether the conglomerate, composed of the debris of the 

 Quincy granite on Adams street, Quincy, mentioned in my paper 

 previously referred to, is the same as the common Roxbury Con- 

 glomerate or not can only be told by actually tracing their rela- 

 tions. The former conglomerate, however, resembles the carbon- 

 iferous conglomerate of the Norfolk basin, as described by Messrs. 

 Crosby and Barton. 2 



It is well known that many writers have held that the Roxbury 

 conglomerate is of Carboniferous age, but of this proof is as yet 

 wanting, although from the evidence obtained to the southward 

 (Norfolk basin) the view is not improbable. 



It may be concluded from the above evidence that the Roxbury 

 conglomerate overlies part of the argillite unconformably, while 

 part of the latter is of the same age and a constituent portion of 

 the conglomerate. This explains and unites the discordant obser- 

 vations and views of the various local geologists; the truth in 

 this case, as in many others, lying partly with both sides, wholly 

 with neither. Further it is possible that two or more conglom- 

 erates exist here of different ages. It may be mentioned that 

 quartzite, like that found so commonly in the conglomerate, 

 has been brought up in fragments by the diabase and is also 

 found by boring in some parts of the Boston basin. 



So far as we have any evidence at present the oldest surface 

 rocks in this basin are the argillites and the schists of allied char- 

 acter. Of these we only know the age of a very small area in 

 Braintree and Quincy. Through these schists and argillites have 

 been protruded immense quantities of eruptive rocks in the form 



1 Occas. Papers, Bost. Soc. Nat. Hist., 1880, in, 238-244. 



2 Airier. Journ. Sci., 1880 (31, xx, 420), 



